1. Field of the Invention
The present invention generally relates to a photoelectric tristimulus colorimeter and, more particularly, to a calibrating system in the photoelectric tristimulus colorimeter for calibrating the spectral sensitivity.
1. Description of the Prior Art
As standards for expressing color in term of quantified values, the CIE (Commission Internationale de l'Eclairage) has recommended, in 1931, the standard colorimetric observer in which the tristimulus values of the radiant flux coming from a given point or area in space and subsequently evoking a sensation of color after having irradiated the central part of the observer's retina exhibit such respective spectral sensitivity curves as shown by solid lines 1, 2 and 3 in the graph of FIG. 6. This signifies that a measurement of the color of an object using three photodetector systems having respective spectral sensitivities duplicating closely the CIE standard color-matching functions, that is, the tristimulus values, can provide a clue as to the color perceived by an observer when the latter receives the color stimuli.
To quantify the color, it is a general practice to utilize a spectrophotometer for the determination of the tristimulus values by measuring the spectral distributions of the radiant flux from a test piece, multiplying the respective measurements of the spectral radiant flux distributions by color-matching functions of the tristimulus values and integrating the resultant products individually. However, as a handy method for the quantification of the color, the use is made of a photoelectric colorimeter in place of the spectrophotometer, which colorimeter makes use of three light receiving elements having respective spectral sensitivities so chosen as to duplicate the spectral tristimulus values for the measurement of the radiant flux coming from an object to give a unique point in the chromaticity diagram.
The present invention pertains to the photoelectric tristimulus colorimeter. The accuracy with which the photoelectric tristimulus colorimeter can give accurate measurements depends on how and to what extend the spectral sensitivities of the three photodetector systems used therein can duplicate the CIE standard color-matching functions as closely as possible. However, the standard color-matching functions recommended by the CIE are not the functions that give respective peak sensitivities at particular wavelengths, but are continuous functions drawn for a wavelength range having a certain bandwidth containing three waveforms having different peak wavelengths which overlap with each other. Therefore, it is not easy to design each light receiving element of the photoelectric tristimulus colorimeter so as to duplicate the CIE standard color-matching functions and an error tends to be pronounced in sensitivity, which error may eventually result in an erroneous indication of the color through a display unit. A deviation in spectral sensitivity is found not only in the light receiving elements of an identical manufacturing lot, but also in those used in different colorimeters and, therefore, an error in measurement tends to be found among the colorimeters.
Various means have hitherto been suggested to compensate for the error. According to the prior art, the use has been made of a plurality of reference samples for calibration purpose each having a known chromaticity. In this instance, the measurement is corrected by determining outputs from the respective light receiving elements relative to various colors, calculating the ratios of actually measured values of the outputs from the light receiving elements relative to reference values corresponding to the known chromaticities of the reference samples (Reference value/Actually measured value) to give calibration constants, storing the calibration constants for the various colors in the form of a table, and selecting an optimum one of the calibration constants from the table which corresponds to the actually measured chromaticity of the sample for the actual correction of the measured value according to the selected calibration constant.
However, the conventional method has a problem in that the optimum calibration constant cannot be properly selected where the chromaticity of the sample measured lies intermediate between one calibration point and another calibration point since the calibration points are of a definite number. In such case, the error tends to occur and, also, since a reference reflector member is used in the determination of the calibration constant and various component parts of illuminating and light receiving systems intervene, effects vary depending on the presence or absence of gloss and a surface condition of an object to be measured. In an extreme case, the error tends to be amplified. While the calibration constant is varied depending on the point in the chromaticity diagram, the prior art method is not a method of correcting the spectral sensitivity curve itself and, therefore, no sufficient effect cannot be obtained.
While the spectral sensitivity characteristics of the light receiving system including filters used to divide rays of light to be measured into three primary color components and associated light receiving elements are required to duplicate the color-matching functions of the CIE standard observer, a deviation from the standards tends to occur in the actually manufactured colorimeter.